(1) Field of the Invention
The present invention relates to a driving method for a solid-state imaging device having pixels arranged in a matrix form, and a solid-state imaging device.
(2) Description of the Related Art
Video cameras and digital still cameras tend to grow in demand year after year. Such growth in demand widens the range of users. Not only old users but also new users require an image having a wide dynamic range. Therefore, it is necessary to develop a solid-state imaging device, in particular, a CCD (Charge Coupled Device) solid-state imaging device for use in a camera, so as to satisfy such requirement of the users.
Hereinafter, a typical CCD image sensor is described. FIG. 1 shows a structure of the typical CCD image sensor. The CCD image sensor includes photodiodes 901 arranged in a matrix form, vertical CCDs 903 each disposed at a left side of the photodiode 901 to serve as a vertical charge transfer path, transfer gates 902 each connecting between the photodiode 901 and the vertical CCD 903, a horizontal CCD 907 disposed on lower ends of the vertical CCDs 903 to serve as a horizontal charge transfer path, and an output unit 908 outputting a voltage value corresponding to electric charges externally from the CCD image sensor. The photodiode 901 converts incident light into electric charges. The transfer gate 902 transfers the electric charges obtained through the photoelectric conversion by the photodiode 901 to the vertical CCD 903. The vertical CCD 903 transfers the electric charges transferred from the transfer gate 902 to the horizontal CCD 907. The horizontal CCD 907 transfers the electric charges transferred from the vertical CCD 903 to the output unit 908. The output unit 908 converts the electric charges into a voltage, and then outputs a voltage value obtained through such conversion externally from the CCD image sensor.
Next, a cause for occurrence of smear is described.
FIG. 2 shows a sectional view of a pixel in a typical CCD solid-state imaging device. In the solid-state imaging device, a photodiode unit (photoelectric conversion unit) 4 includes an n-type silicon substrate 10, a p−−-type well region 11, an n-type charge accumulation region 12 and a p++-type region 13. Also in the solid-state imaging device, a vertical CCD unit 5 includes an n-type buried channel region 14, a p−-type region 15 formed below the n-type buried channel region 14, and a gate electrode 16 such as a polysilicon film. The vertical CCD unit 5 has a function of transferring signal charges obtained through photoelectric conversion to a horizontal CCD unit.
A transfer gate unit 17 is formed between the photodiode unit 4 and the vertical CCD unit 5. A p+-type channel stop region 18 is formed at a position opposite to the transfer gate unit 17 with respect to the photodiode unit 4. In other words, the photodiode unit 4 is interposed between the transfer gate unit 17 and the p+-type channel stop region 18. A gate insulating film 19 having a lamination structure of a silicon oxide film and a silicon nitride film is formed on the n-type buried channel region 14, the transfer gate unit 17 and the p+-type channel stop region 18. A light shielding film 8 such as a tungsten film is formed on the gate electrode 16 with an interlayer insulating film 20 interposed therebetween. A reflection preventing film 21 is formed on a surface of the photodiode unit 4.
Smear occurs due to the following reason. That is, upon accumulation of electric charges obtained through photoelectric conversion, the electric charges flow into the vertical CCD unit 5 without being accumulated in the photodiode unit 4, so that a false signal is generated. This fact is described with reference to FIG. 2. It is considered that smear occurs due to the following four reasons in general. (i) Light transmits through the light shielding film 8, reaches the vertical CCD unit 5, and is converted into electric charges in the vertical CCD unit 5, so that smear occurs. (ii) Incident light is partially leaked from an interface between the light shielding film 8 and the gate insulating film 19, and then transmits to the vertical CCD unit 5 while reflecting at multiple paths in the light shielding film 8, between the gate insulating film 19 and the gate electrode 16, and between the gate insulating film 19 and the light shielding film 8. Thereafter, the light is converted into electric charges in the vertical CCD unit 5, so that smear occurs. (iii) Electric charges obtained through photoelectric conversion at an exterior of the photodiode unit 4 are diffused, and then reach the vertical CCD unit 5, so that smear occurs. (iv) Electric charges obtained through photoelectric conversion at a recombination region in the p++-type region 13 on the surface of the photodiode unit 4 are transferred or diffused by a weak electric field. Thereafter, the electric charges reach the vertical CCD unit 5, and then are detected as a false signal, so that smear occurs.
It is considered that smear occurs due to the aforementioned reasons. In response to formation of a finer pixel, there are constantly required countermeasures against smear. This is based on the following reasons. In the case where a pixel size is simply shrunk, a width of the transfer gate unit 17 shown in FIG. 2, that is, a distance between the photodiode unit 4 and the vertical CCD unit 5 becomes short. Thus, a light transmission distance in the aforementioned case (ii) becomes short, for example. Consequently, an amount of the light to be absorbed upon transmission is decreased, and the light readily reaches the vertical CCD unit 5, resulting in occurrence of smear. In addition, an electric-charge diffusion distance in the aforementioned case (iii) or (iv) becomes short. Consequently, the electric charges readily reach the vertical CCD unit 5, resulting in occurrence of smear. As described above, the simple shrinkage in pixel size causes occurrence of smear.
In order to suppress occurrence of smear, there is known the following technique. That is, by provision of a packet for reading out normal signal charges (signal charges plus smear charges) and a packet for reading out only smear charges, the smear charges are subtracted from the normal signal charges (signal charges plus smear charges) (refer to, for example, Japanese Unexamined Patent Application Publications No. 2005-328212 and No. 2001-119629).
Japanese Laid-Open Patent Application No. 2005-328212 discloses a technique for reading out both signal charges and smear charges from a pixel and reading out only smear charges from a different pixel, and then subtracting the smear charges from the signal charges and the smear charges.
Japanese Unexamined Patent Application Publication No. 2001-119629 discloses a technique for reading out both signal charges and smear charges and reading out only smear charges from one pixel at different timings, and then subtracting the smear charges from the signal charges and the smear charges.